In a typical cellular communications system there are many system parameters that should be adapted to the actual situation, and to each other, in order to make the system work as efficiently as possible. For example, certain physical layer parameters may be adapted, such as frame structure, pilot distribution and multiple antenna transmission mode (beamforming, MIMO, space-time coding and others).
In a future system it may also be possible to select among different radio interfaces in a base station depending on the deployment scenario. For example, in micro or pico cells WLAN may be used and in large macro cells WCDMA or GSM/EDGE may be used.
When adapting the system parameters the operator will try to match these to situations that may occur in a certain cell. Different types of planning tools exist where certain parameters such as, for example, radio propagation models, are selected to match the anticipated environment, for example, in terms of an assumed worst case scenario or in terms of typical conditions in this environment. The actual communication in the cell may be very different from the initial assumptions made when deploying the system. Also, if a cell that has been designed for a particular environment and that environment is changed, e.g. new buildings, roads or other objects appear in the surroundings, the parameters may be changed to reflect the new situation. Either the communication in the cell will not work very well, or the communication will be unnecessarily complex. Variations in user behaviour over time can also cause the experienced channel conditions to deviate further from initial assumptions, for example, during busy hour.
Multipath propagation of radio waves in a wireless communication system results in a pattern of standing waves where minima are encountered when the vector sum of all waves is zero or close to zero. A mobile unit moving through this standing wave pattern will experience rapid signal variations, fading, that present a challenge to upholding and optimizing the transmission and reception of information. The multipath fading can be experienced in both the time domain and in the frequency domain. Multipath fading can be time-selective or frequency-selective.
Time-Selective Multipath Fading:
Movement of the receiving antenna through the standing wave pattern will result in signal variations in time. By reciprocity, the same variations will be observed if the direction of transmission is reversed, i.e. the moving receiver becomes a moving transmitter and the stationary transmitter becomes a stationary receiver. Furthermore, even if both transmitter and receiver are stationary, movement and changes in the surroundings of the two may result in changes to the standing wave pattern and hence time variations of the received signal. All types of movement give rise to what is referred to as time-selective multipath fading, or time selectivity.
Frequency-Selective Multipath Fading:
The phase of each radio wave is a function of the path length expressed in wavelengths. If the frequency is shifted, the phase of each radio wave may also be shifted, and the standing wave pattern is changed. Thus, at a given time instant, the received signal will have fading variations over the frequency band that is referred to as frequency-selective multipath fading or frequency selectivity.
In addition, multiple antennas may be used for transmission and/or reception of the radio waves. Properties of the antenna arrangement such as relative positions, radiation patterns, mutual coupling and polarization will result in different weighting and phase shifts of the radio waves at different antennas. Hence, the standing wave pattern associated with one transmitting antenna may be partially or fully independent of that associated with another transmitting antenna. By reciprocity, the same hold for different receiving antennas. Thus, different signal strengths may be encountered for different antennas, which will be referred to as antenna selectivity.
The time and frequency selectivity of the wireless communication channel presents a challenge to upholding efficient communication. Various methods have been devised to utilize the selectivity and improve the system performance; examples of such methods are coding, diversity, scheduling and Automatic Retransmission Request (ARQ) or hybrid ARQ.
Coding and diversity add redundancy to avoid the loss of information caused by fading dips, while scheduling utilizes channel knowledge to distribute the information over time and frequency to avoid times or frequencies in which the channel conditions are poor in terms of poor signal strength or high interference.
All of these methods will improve information transfer under certain channel conditions that are typical for the operation of wireless systems. However, all of the described methods also have limitations depending on the time and frequency selectivity of the channel. When the channel variations with time or frequency are slow compared to the extent of the transmitted data block the methods cannot provide resilience towards channel variations or C/I variations. On the other hand, when the channel variations are very fast a large amount of overhead information is needed for the receiver to be able to estimate the channel, leaving less room for the transfer of information. Frequency selectivity also introduces inter-symbol interference (ISI), which may lead to a need for channel equalization. This will require more complex and costly receivers.
A well known method for creating additional selectivity in wireless systems is to use more than one antenna at either transmitter or receiver or both. This creates multiple channels that may be more or less decorrelated, allowing the use of redundancy or scheduling over the antenna domain. The drawback of this solution is added complexity and cost, and often the need for more overhead signalling.
Adding time or frequency selectivity to the wireless channel using multi-antenna transmission of a cellular communications network is known per se. For example, in TDMA systems, delay diversity can be applied uniformly in a cell. In this case, two or more antennas are used to transmit delayed replicas of the same signal. At the receiving antenna, the delayed replicas will be superimposed and give rise to an increased time dispersion compared to the single antenna case. In this way increased frequency selectivity is achieved. If the frequency selectivity is increased too much increased inter-symbol interference and system degradation will result.
Two or more antennas may also transmit the same signal with different time varying amplitudes and/or phases, to achieve time selectivity. This technique, in combination with channel dependent or quality dependent scheduling, has been termed opportunistic beamforming. The additional time selectivity introduced in this way is beneficial in slowly varying channels but can cause problems if the channel is already varying rapidly.
A combination of time and frequency selectivity has also been described, where different time variations are introduced in different segments of the frequency band.
When the channel does not provide sufficient time and frequency selectivity the use of an artificially created time and frequency selectivity can lead to large improvements in system capacity and throughput. On the other hand, if the channel for a certain user already provides sufficient selectivity, there is no additional gain from artificially creating more selectivity in the time or frequency domain. It may even be harmful if the selectivity exceeds that required for the system to operate in an optimal way. In this case time and frequency variations may be so rapid that they cause degradation due to, for example, inter-symbol interference or channel estimation errors. Given a certain system configuration there exists a region of selectivity in which the system performance is improved compared to outside this region. Prior art solutions only enable this region to be moved, but not extended.
Also, in a multi-user system each user experiences individual channel conditions. For some users the performance would benefit from artificial selectivity while others might be harmed by it. Selecting the appropriate amount of artificial selectivity becomes a difficult design choice that may have undesired consequences if the experienced channel conditions differ from the expected. A related design choice is the selection of the maximum time dispersion and mobile speed at which a certain system performance should be required. The artificially induced time and frequency variations must be taken into account, which effectively lowers the maximum possible rate of channel variations.
Yet another problem with the existing solutions is that both instantaneous and average channel characteristics will differ between the uplink and the downlink, as the artificial selectivity is only induced in one of the links. Therefore, methods that rely on the similarity of the characteristics between uplink and downlink will suffer from degraded performance. For example, in a time-division duplex (TDD) multiplex system the channel is usually assumed to be identical in both directions. This will not be the case if artificial selectivity is introduced in one of the links. Another example could be where a transmitting unit can choose from two possible coding/interleaving schemes, one best suited for rapid time variations of the channel and the other best suited for slow channel variations.
Communication systems can be designed to be flexible in how the transmitted information is spread over time and frequency, both to be able to adapt to the coherence time and frequency of the channel and to adapt to the different requirements of different users. Artificially inducing a fixed amount of selectivity may limit the benefit of this flexibility, for example, if one user requires a large part of the available frequency band. Too much frequency selectivity could lead to this user being scheduled on many small non-contiguous frequency segments, whereas with less frequency selectivity the user could have been scheduled on one contiguous segment. The latter situation would require less signalling overhead.